Denver ? Roaring toward the finish, Barack Obama presided Sunday over two Colorado rallies that together drew about 150,000 people, a startling turnout in a key swing state.

In Denver, the city where he claimed his historic presidential nomination, Obama stepped on stage and seemed surprised at his own following. He saw an estimated crowd of more than 100,000 people – the largest U.S. rally to date in an Obama campaign full of them.

“Goodness gracious,” Obama said as peered at the human mass in Civic Center Park.

Smelling victory, supporters even lined the steps of the Capitol, which was so far away from the stage that the people there needed binoculars just to hope to see Obama.

The setting, on a sparkling day in this battleground state, said perhaps more than Obama did in his actual speech. It rippled with the kind of enthusiasm found at victory rallies.

The location of a later rally – a Colorado State University lawn known as “The Oval” – suggested Obama’s possible future workplace. He spoke to an estimated 45,000-50,000 people at the Fort Collins event.

Obama’s campaign is capitalizing on the scope of such rallies to get people to cast votes early, permitted in Colorado and more than two dozen other states.

“How many people have early voted?” Obama said, eliciting cheers from people bundled up in fleece. “That’s what I’m talking about. No point in waiting in lines if you don’t have to. You know who you’re going to vote for.”

Still, wary of complacency or overconfidence, Obama keeps warning supporters that they must work, fight and even struggle for the rest of the campaign.

Obama even ended his day in Colorado by calling voters directly.

At an unscheduled stop at a campaign office in Brighton, northeast of Denver, Obama sat down and called about a dozen unsuspecting registered voters. He shuffled from one call to the next as thrilled campaign volunteers kept placing calls and handing him cell phones.

Based on what reporters could hear from Obama’s end of the conservation, all of the calls went well for him. He then told volunteers to keep working through Election Day.

“It’d be terrible if we just kind of let it slip away in that last few days,” he said.

Polls put Obama ahead in Colorado with the number of campaign days remaining now down to single digits.